Detection of outbound message signals in a power line two-way communications system

ABSTRACT

Improvements in the detection of TWACS outbound message signals. A first improvement involves matching some (or all) of the intermediate points in an outbound preamble occurring between bits of the preamble currently being detected. This reduces the possibility of a false synchronization and therefore decreases the probability of missing outbound message signals. A second improvement is to require some or all of the known preamble bits to exceed a predetermined threshold where both the thresh-old and which bits are adjustable. An additional approach is using 4-8 additional buffers in a transponder to detect preamble patterns in the outbound message. Each half cycle of the outbound message waveform requires entering a bit only into the buffers active for the particular frame of reference in which the message is being transmitted, since only buffers for that frame of reference are employed. The process continues until all bits specified to be sent, based on the length of the outbound message, are extracted. A CRC is then performed for the message. Using this method eliminates the problem of inbound messages being detected as outbound messages, and the resulting “blindness” of the transponder. It further makes the transponders less sensitive to noise which currently causes the transponder to detect a preamble when there is none, resulting in a valid outbound message being missed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the United States National Stage under 35 U.S.C.§371 of International Application Ser. No. PCT/US2008/060731 having anInternational Filing Date of Apr. 18, 2008, and is related to and claimspriority to U.S. Provisional Patent Application No. 60/913,612, filed onApr. 24, 2007 which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

N/A

BACKGROUND OF THE INVENTION

This invention relates to a two-way automated communications system(TWACS) used by electrical utilities for sending messages from theutility to an end device, and for receiving and processing responsesfrom the end device containing information relating to matters such ascurrent energy consumption, power outages, etc. More specifically, theinvention sets forth a method for improvement in the detection foroutbound messages sent by the utility.

TWACS messages, which can be categorized as either outbound messages(OM) or inbound messages (IM), are sent and received over a selectedphase (φ) of the electrical distribution system. When an outboundmessage is sent, the transponder detects the message and validates itusing channel coding. As discussed hereinafter, synchronization to theoutbound message signal involves matching a 9-bit preamble of themessage against an expected bit pattern. It will be understood thatsynchronization does not necessarily mean the message detected by thetransponder will pass error detection as part of the channel coding.Furthermore, because the entire message has not yet been received at thetime of synchronization, the transponder must continue to detect bitsuntil the end of the message is received. If the transponder erroneouslydetects what appears to be a preamble, it will attempt to detect thebits following the preamble, even if no signal is present. This canresult in the missed detection of an outbound signal that occurs whilethe transponder is in the bit detection state.

Sometimes an erroneous preamble detection can be triggered in atransponder by random noise on the power line, but a far more likelycause is TWACS inbound signals. There is sufficient similarity betweenTWACS inbound signals and TWACS outbound signals that an inbound signalcan contain a bit pattern that matches the preamble of an outboundmessage. Depending on system configuration, it is possible for suchpatterns to occur immediately before an outbound message is sent overthe same phase as a particular transponder.

Similarly, a transponder can miss a TWACS outbound message when it ispreceded by another outbound message. Consecutive outbound messages canbe sent on the same phase or different phases. If a transponderincorrectly synchronizes to the first outbound message (due to othernoise sources) the transponder is more likely to miss the secondoutbound message as well.

This problem is compounded because once a transponder synchronizes to abit pattern that matches the outbound preamble, the transponder stopslooking for any other preamble patterns. It will be understood that thebit pattern detected could be incorrect as well as correct. As thetransponder continues processing what it now believes is an outboundtransmission based upon synchronization to an incorrect bit pattern, ablind spot (time period) is created in the transponder. If a trueoutbound message is transmitted on the phase of the transponder duringthis period, it will miss the message because it is not looking for itduring that time. The blind spot may last anywhere from a few bytes ofTWACS outbound message time (for the shortest valid message), up to themaximum message size of 31 bytes. This corresponds to a range in time ofbetween 2.13 seconds and 16.50 seconds.

While inbound signals are a common source of false outbound preamblesthat cause the outbound detector to miss true outbound signals, theproblem can also be triggered by power line noise. When no signals arepresent and the transponder is hunting for a preamble, there is acertain probability that any random noise sequence will match theoutbound preamble pattern. Again, this may result in the transponderprocessing what it believes is the outbound message without furtherwatching for a true outbound message.

BRIEF SUMMARY OF THE INVENTION

The present disclosure is directed to improvements in a method of thedetection of preamble patterns in TWACS outbound messages which benefitthe likelihood of detecting a true TWACS outbound message. The firstimprovement includes matching some (or all) of the intermediate pointsin an outbound preamble waveform that occur between those bits currentlybeing detected, in order to reduce the possibility of falsesynchronization and therefore decrease the probability of missingoutbound messages. Tests have shown that this improvement significantlyreduces the relative number of missed outbound messages.

Another approach is to utilize additional (2-8) buffers in a transponderfor processing outbound messages concurrently, each buffer adding about35 bytes of RAM each, while requiring a minimal amount of additionalprocessing power. When an outbound message preamble is detected, thesynchronization detector continues to search for additional outboundmessage preambles, and if additional preambles are detected, theassociated bits are buffered using the additional buffers. The processcontinues until all bits associated with each message's preamble arecollected. A cyclic redundancy check (CRC) is then performed todetermine the validity of the collected bit pattern. Using this methodsignificantly reduces the duration and frequency of any “blindness” ofthe transponder because the preamble searching continues until allbuffers are full, even if an erroneous preamble is detected.

Other objects and features will be in part apparent and in part pointedout hereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The objects of the invention are achieved as set forth in theillustrative embodiments shown in the drawings which form a part of thespecification.

FIG. 1 is an illustration of a noisy outbound preamble sequence afterpassing through a filter;

FIG. 2 is a graph showing the probability of false synchronization ofinbound signals for each of the 63 possible combinations for one of thesix (6) channel sets used for TWACS inbound signaling as simulated undersevere operating conditions;

FIG. 3 illustrates the performance of a zero/non-zero value ratio testfor various threshold values;

FIG. 4 is a simplified representation of a TWACS used by a utility; and,

FIG. 5 is a flow chart of the method of the invention.

DETAILED DESCRIPTION OF INVENTION

The following detailed description illustrates the invention by way ofexample and not by way of limitation. This description clearly enablesone skilled in the art to make and use the invention, and describesseveral embodiments, adaptations, variations, alternatives and uses ofthe invention, including what is presently believed to be the best modeof carrying out the invention. Additionally, it is to be understood thatthe invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or carried out invarious ways. Also, it will be understood that the phraseology andterminology used herein is for the purpose of description and should notbe regarded as limiting.

The preamble of an outbound TWACS message consists of a 9-bit pattern011100100, or 0E4 hexadecimal. The ones and zeroes are represented byputting a pulse on the first or third half cycle of the AC waveform nearthe zero crossing points. For a given outbound message all pulses areplaced on either positive or negative half cycles. Accordingly, a TWACSoutbound signal detector looks at all of the zero crossings. Thefollowing is a representative pulse pattern detected by a receiver. Thetop row is bit number; the middle row is the binary sequence; and, thebottom row is the pattern of pulses. A “1” represents the presence of apulse and a “0” the absence thereof:

bit 1 2 3 4 5 6 7 8 9 binary 0 1 1 1 0 0 1 0 0 seq. pulse pat. 1 01000100 0100 0001 0001 0100 0001 0001

It will be understood by those skilled in the art that the leading zerosof the first bit (bit 1) are ignored in the above representation. Toremove the AC waveform from the signal being processed, the receiversubtracts data collected one cycle earlier in the processing sequencefrom the waveform currently being processed. This can be done as afiltering operation, using filter coefficients of [1, 0, −1]. The outputof this operation, as performed over the entire preamble is:

-   -   1000 −1010 −1010 −100010 −101000 −100010 −1010x        The x at the end of the pattern is indeterminate because it        depends on the value of the next bit in the message sequence. It        will be noted that the scale of the signal has not been        considered, so that the values 1 and −1 are essentially        “placeholders” for some arbitrary value that is either positive        or negative. The sequence has a total length of 35 bits        including the indeterminate bit at the end of the sequence.

FIG. 1 illustrates a noisy version of the preamble sequence, afterfiltering. As discussed hereafter, the bit rate is one bit for everyfour half-cycles; so four possible synchronization points areillustrated and labeled numerically in FIG. 1. In FIG. 1,synchronization point 1 is the correct alignment for the message beingprocessed. If resulting bits are now extracted from the preamble, theresulting sequence is:

-   -   1 −1 −1 −1 1 1 −1 1    -   1        where a positive value (represented here by a 1) maps to a        binary 0, and a negative value (represented here by a −1) maps        to a binary 1. The outbound detection method previously used        checks to verify if this pattern matches the expected pattern,        and also checks to determine whether the bits exceed a        predetermined threshold of signal strength. This threshold is        illustrated in FIG. 1 by the gray region in the middle of the        Figure, and is considered to be a “zero” region. That is, a        signal falling within this region is assumed not to be an        out-bound signal. This can be thought of as a quantization of        the signal to one of 3 values, corresponding to 1, 0, and −1. In        the past, the detector has not required that all preamble bits        meet the threshold requirement, but recent tests have shown that        this is effective in reducing false preamble detections.        Further, using only every fourth value to evaluate the validity        of a possible synchronization sequence ignores valuable        information occurring between those values. As shown in FIG. 1,        while reference frame 1 is the correct synchronization point,        reference frames 2 and 4 are both near zero, and reference frame        3 contains a mixture of zero and non-zero values. Since all of        these values are processed before the correct synchronization        point is ascertained, an optimal false preamble rejection        algorithm should take these values into account.

If intermediate values are included in the received preamble sequence,what essentially is being accomplished is trying to match a ternarysequence of length 33 (ignoring the indeterminate part at the end).However, because the amplitude of the signal is not known in advance, anoptimal threshold between what constitutes a 0 and a 1 or −1 can only bedetermined adaptively. A simplified test to do this, for example, wouldbe to take only those points where a 1 or −1 is expected to occur. Byonly requiring a 9-bit pattern match, one is effectively only matching 9points out of 33. But, by requiring all non-zero locations to alsomatch, 13 points are now being matched instead. Were completely randomnoise being processed, this would result in a reduction in falsesynchronizations by a factor of 2⁴ or 16. However, this is notnecessarily true if the noise is caused by an inbound TWACS signal,which is not random.

Because inbound signals can easily be seen by the outbound detectors intransponders of the adjacent phases, it is important to be able tocorrectly reject patterns that may look similar to outbound messagepreambles, but are in fact part of inbound messages. After passing aninbound signal (or the sum of multiple inbound signals from multiplesources) through an outbound message filter, it is still possible forthe result to match the non-zero portions of the preamble. However,because the pulse patterns in the inbound message signal are differentfrom those in an outbound message, they contain non-zero information inplaces where the outbound message pattern should be zero. Therefore, away of rejecting false synchronizations on inbound signals is to enforcesome type of test on the other indices. One example of how to do this isto investigate the ratio of the signal energy level in places where azero is expected to be, to the energy level in places where a one isexpected. To define such a ratio, for the results of the filteringoperation set forth above, let Z represent the set of indices thatcorrespond to the places where there are expected values of zero; i.e.,

-   -   Z={2, 3, 4, 6, 8, 10, 12, 14, 15, 16, 18, 20, 22, 23, 24, 26,        27, 28, 30, 32}        Similarly, let N represent the indices that correspond to the        places where there are expected non-zero values:    -   N={1, 5, 7, 9, 11, 13, 17, 19, 21, 25, 29, 31, 33}        If d_(i) is the absolute value of the received signal at index        i, let R represent the ratio of zero to non-zero signal        strength. That is:

$\begin{matrix}{R = \frac{\sum\limits_{i \in Z}\; d_{i}}{\sum\limits_{i \in N}\; d_{i}}} & (1)\end{matrix}$Now, if R exceeds a predetermined threshold value, it will be rejectedas not a true outbound message preamble. Based upon the simulationresults presented below, an acceptable range of threshold values is inthe range of 0.6 to 1.0.

This new synchronization method was tested on simulated inbound andoutbound message signals. The noise level selected was relatively highin order to simulate severe operating conditions. In addition, theoutbound signals were subjected to an impulse response that createdleakage into the next two half cycles of the waveform, with a 20%leakage after one half cycle, and an additional 10% leakage after twohalf cycles. These conditions resulted in a 3% probability of anoutbound message signal being missed altogether. This value is higherthan what is generally seen in practice.

The inbound waveforms were generated for all possible combinations forthe 6 channels in the TWACS inbound channel scheme, and include 6permutations of 1 channel, 15 permutations of 2 channels, 20permutations of 3 channels, 15 permutations of 4 channels, 6permutations of 5 channels, and one permutation for all 6 channels. Thisresults in a total of 63 possible combinations.

FIG. 2 shows the probability of a false synchronization on inboundsignals for each of the 63 combinations. The plot contains the meannumber of false synchronizations per inbound message for the traditional9-point pattern matching, and the proposed 13-point pattern matching. Ina few instances in FIG. 2, the reduction in false synchronizationsachieved by the 13-point matching technique approaches the theoreticallimit of 16 for a completely random input. On the other hand, there aretwo instances, channel 6 and channel 14, where all cases of matching the9-point pattern also match the 13-point pattern. This is a peculiarityof the particular pulse patterns for those channels, although theprobability of a false synchronization is already relatively low forthem. Overall, the average reduction in false synchronizations isapproximately 60%.

FIG. 3 shows the performance of the zero/non-zero value ratio test forvarious threshold values with the curve of interest being indicated A.The other two curves B and C are explained hereinafter. In FIG. 3, oneaxis represents the probability of erroneously accepting data in aninbound message as an outbound preamble, and the other axis representsthe probability of erroneously rejecting a true outbound preamble. Undercleaner (i.e., less noisy) signal conditions than those used in thesimulation, this curve tends to shift to the left as viewed in FIG. 3.

With the threshold set to a relatively conservative value of 1.0, theprobability of falsely rejecting an outbound message was 0.1%. It willbe noted that, at this noise level, the probability of missing apreamble due to just noise is 17%. At this level, only 6% of the placesin inbound messages that would have been detected using traditionalsynchronization methods were able to pass both the 13-point patternmatch and the zero/non-zero ratio test. With the noise level set so thatexisting detection methods missed 3% of outbound preambles, theprobability of falsely rejecting an outbound message was reduced to0.01%, and the inbound message acceptance percentage increased to 5%.This is a reduction, by a factor of 20, in the number of false preamblepatterns that are accepted. The remaining false alarms can besubstantially eliminated by use of a second detector; or, if resourcesdo not permit, the threshold could be reduced to, for example, 0.9.

This false preamble rejection method has some elements that requiresubstantially more memory than has been used in the past, which couldmake implementation in some transponders impractical. The main problemin this regard is that the ratio test to determine whether or not toreject a possible preamble is essentially a ratio of the outputs of twofinite impulse response (FIR) filters. Implementing such filtersrequires that data be stored in a memory for the entire length of afilter, about 35 half-cycles. Since these filters are replicated severaltimes in the transponder, each additional past value that must be storedfor the filter results in a large increase in memory consumption.

Currently, average signal amplitude is computed for each of the fourpossible synchronization points using a first-order recursive filterwhich is updated at each step using the running average of previouslyentered values, plus the next new value. The averages are used to createa modified zero to non-zero ratio test. For example, let x_(n) representthe input to the filter at half-cycle n. At each n, there are fourfilter outputs, y_(n,1), y_(n,2), y_(n,3), and y_(n,4). These representthe filtered outputs for the four synchronization points illustrated inFIG. 1. It will be noted that if the filtering function is anon-weighted average of the last 9 values, y_(n,1) will contain a largeoutput, y_(n,2) and y_(n,4) will have values close to zero, and y_(n,3)will have a value somewhere in between. Given the above definitions, theideal zero/non-zero ratio could be approximated with the ratio:

$\begin{matrix}{R = \frac{y_{n,1}}{y_{n,2} + y_{n,4}}} & (2)\end{matrix}$which is similar to the original definition of R in Equation (1) above,except that because some terms in both the numerator and denominatorfrom the original definition are contained in y_(n,3), it is removedcompletely from the approximation in Equation (2).

In practice, computing the filter outputs y_(n,1) . . . y_(n,4) asunweighted averages of previous inputs requires significant additionalmemory, so recursive averages can be used as an approximation. A simplefirst-order recursive average does not require any additional memorybeyond the storage of the y values asy _(n) =x _(n) +αy _(n−1).  (3)The value of α is an averaging constant that should be chosen tooptimize rejection of false synchronization patterns. If it is nowassumed that at half-cycle n, the 13-point pattern match is satisfied,then either the zero/non-zero amplitude ratio (Equation 1) or theapproximate zero/non-zero amplitude ratio (Equation 2) is computed andcompared to a predetermined threshold, in order to judge the likelihoodthat the received pattern actually represents an outbound preamble.

It should be noted that equation (1) represents an ideal case whichrequires extra memory, and equation (2) represents a simplified casewhich requires no extra memory. These examples are intended to beillustrative only, and not limiting. For example, in a situation wheresome additional memory is available, but not enough to implementequation (1), a different filtering operation than that illustrated inequation (3) could be implemented that uses additional internal filterstates to more closely approximate the filter response of equation (1).

One problem encountered using IIR filtering is that due to its infiniteimpulse response, the output of the filter will include past resultsthat are older than the synchronization sequence. In FIG. 1, the onlysignal present prior to the outbound message preamble is low-levelnoise, which represents a best-case scenario. The worst-case scenario,when IIR filtering is used in computation of the ratio test, is when avery strong inbound message signal is received immediately prior to thepreamble of an outbound message. For transponders sharing the sameservice transformer, the signal strength of the inbound message signalseen by the neighboring transponders can be many times greater than thatof the outbound message signal. In constructing a worst-case scenariosuch as this for simulation purposes, it is assumed that inbound messagesignals are received on all 6 channels immediately before the outboundmessage signal of interest, and that each inbound message signal has anamplitude 10 times that of the outbound message signal. What this meansis that the exponential decay of the IIR filter must be fast enough todiscard or “forget” the effect of the inbound message signals prior tothe end of the preamble of the outbound message. Empirical tests haveshown that for use with the method of the invention, best performance isobtained using a value of α=0.5. Using this value has an advantage inthat the necessary computations can be performed using a simple bitshift operation.

Simulation results for the modified zero/non-zero ratio test are shownin FIG. 3. As indicated above, for the modified ratio test, a stronginbound message signal immediately precedes each outbound messagesignal. However, it should be noted that this has no effect on thenon-modified ratio test because the inbound message signal has no IIRcomponent. The curve B in FIG. 3 represents the situation where theinbound and outbound message signals are equal in amplitude; and, thecurve C represents the situation where the inbound message signal is 10times stronger than that of the outbound message signal. The relativelysmall gap between the curves indicates that the impulse response of theIIR filter approaches zero relatively quickly. The result is that thereis only a minimal amount of “leakage” of data from previous signals intothe ratio test. While the modified ratio test does not perform quite aswell as the unmodified ratio test, given the substantial reduction inmemory consumption required for the modified ratio test, the resultsnonetheless represent a significant gain over existing outbound messagedetector techniques.

One reason for sub-optimal performance of the reduced-memorymodifications to the false preamble rejection is that non-zerointermediate values observed at synchronization point 3 (y_(n,3)) inFIG. 1 are not included. However, it is still possible with a reducedmemory to test these values to some extent, because the quantizedversion (to a ternary value) of this data is still stored. Now, insteadof matching the sign of 13 points in the data as a preliminary rejectioncriterion, the method can require that all 33 data points also fall intothe correct bit strength range. That is, all values (and only thosevalues) that are expected to have non-zero bit strength are required toexceed the predetermined threshold. A drawback to this method, at thistime, is that the threshold is currently not adaptive and is thereforeset relatively high. It is possible, in some instances, that a weaksignal with a high level of noise may cause one of the non-zero pointsto be not strong enough, or that a short impulse noise source couldcause a zero-valued point to exceed the threshold. Because of this, itis reasonable to allow for one or two processing errors, but tests havenot yet been performed to determine what would be acceptable.

In summary, a received signal is assumed to be a valid synchronizationpattern when the following criteria are satisfied:

-   -   a) All 13 non-zero values have the correct sign;    -   b) A predetermined subset of the non-zero values are expected to        exceed a predetermined threshold    -   c) Most, or all, of the 33 values fall into the correct range        with regard to the threshold for bit strength; and,    -   d) The ratio R as defined in Equation (1) or Equation (2)        exceeds the prescribed threshold.

When a TWACS outbound message occurs on a different phase of the ACpower line, the situation is referred to as “outbound crosstalk”.Currently, transponders accept the outbound message rather than blockit, because they cannot determine the difference between a TWACSoutbound message on a different phase and a message on their own phase.If a transponder detects an outbound message that is actually crosstalk,it currently stops looking for another valid preamble in the outboundmessage for the calculated inbound message time period. If the TWACS'communication equipment (SCE) sends an outbound message on the samephase, that signal will be missed because the transponder is “blind”during the inbound message time period on the adjacent phase.

Since a basic problem is that many transponders are designed (orprogrammed) to stop searching for a new preamble pattern in an outboundmessage once a preamble pattern is found, another solution to theproblem is to program the transponder to continue searching for preamblepatterns even after one has been found. As discussed above, detectors incurrent transponders stop searching for a preamble pattern once one isfound because the transponder has a limited amount of memory, and thereis a limited amount of processing power available to the microprocessorsused in the transponders. With the introduction of newer transpondershaving more powerful microprocessors, more memory, and greaterprocessing speed, these limitations are relaxed.

Current outbound detectors employ a set of tables to track detectedoutbound message bits for fourteen (14) different detection rangesacross four (4) different possible half-cycle alignments. Each bit of anoutbound message signal is coded using four (4) consecutive half cyclesof the AC waveform, which results in four (4) different frames ofreference on which an outbound message OM can be found, so the detectormust track each of the four (4) frames of reference. For each half cycleof the waveform, only one of the four (4) frames of reference isevaluated, so the bit rate for each frame of reference is one (1) bitfor four (4) half cycles.

The tables also track the preceding eight (8) bits of a signal, both asto whether the bits appear as a 1 or a 0, as well as the signal strengthof each bit. A 1 in the map indicates a signal strength greater, forexample, than a threshold value of 25 μsec; while, a zero indicates asignal strength below this threshold. The tables also track a runningaverage of the bits. In this regard, the signal strength of each bit isadded to the average signal strength of the preceding bits, and theresulting sum is then divided by 2. This is done for all the ranges.When an outbound preamble is detected on one or more of the ranges forthe frame of reference which is being evaluated for that particular halfcycle, then processing for that frame of reference is halted. Thedetection range having the greatest running average signal strength isnow selected as the best range, and from there on, for the remainder ofthe outbound message signal, only data for that range and frame ofreference is calculated. If the signal strength for a bit is positive,it is detected as a 0; if the signal strength for that bit is negative,it is detected as a 1. The bits are then shifted into an outboundmessage buffer of the transponder until the number of bits specified inthe outbound length portion of the message have all been detected. Aseach bit of the outbound message is detected, the detector then usesthat information to determine how many more bits need to be receivedbefore attempting to validate the message using a CRC check code inorder to generate a checksum for the outbound message used to detecterrors in the message.

Using the method of the invention, no additional tables are needed andthe transponders need only to continue searching for a preamble on all14 ranges and on all 4 frames of reference. Several additional outboundmessage buffers are now used and these will require additional variablesto describe what they are. One thing that will need to be known is onwhich frame of reference an outbound message preamble was detected for abuffer. Another thing that will need to be known is the detection rangeon which the outbound message preamble was detected. When enough of themessage has been received to know the length of the outbound message,that information will be used to determine when to perform the CRCvalidation on that buffer. Another item required is a bit counter usedto keep track of where the next detected bit goes.

Preferably, 4-8 outbound message buffers are required, with each bufferonly adding about 35 bytes of RAM. The additional processing powerrequired by the additional buffers to detect outbound is minimal. Eachhalf cycle only requires storing a bit into as many buffers as areactive for that frame of reference, and only the outbound messagebuffers for the current frame of reference need be managed. However,even if all 8 buffers were active for the same frame of reference, eachbuffer only needs to have the bit inserted into the buffer, and the bitcount of the buffer updated. The process continues until all bitsspecified to be sent, based on the extracted outbound message lengthinformation, are received. Then, the CRC is evaluated for the message.It is important that calculation of the CRC not impact other processingwhich is on-going, so the searching for outbound preambles continues.Since the CRC check process is time consuming, it is done as each bit isreceived, instead of all at once after the last bit is detected. Thisincrease in processing is proportional to the number of detectors.

Use of this method will significantly reduce the likelihood of TWACSinbound messages being detected as outbound preambles, with theresulting “blindness” of the transponder as previously described. Itwill also make the transponders more resistant to certain types of noisewhich currently cause it to detect an outbound preamble when there isnone, and subsequently miss a valid outbound message. The changesrequired to affect this are relatively minimal, since all that needs tobe selected is the number of additional outbound message buffers to bemaintained. The number of additional buffers could even be variable andso allow a higher number of buffers to be used in transponders having ahigher processing capability; with a fewer number being used the lowerpowered, slower speed units.

In view of the above, it will be seen that the several objects andadvantages of the present disclosure have been achieved and otheradvantageous results have been obtained.

The invention claimed is:
 1. In a two-way automatic communicationssystem used by an electrical utility in which outbound messages are sentfrom the utility to an end device and inbound reply messages are sentfrom the end device back to the utility, the respective outbound andinbound messages being sent and received over the utility's powerdistribution system, a method for enabling a transponder to distinguishan outbound message from an inbound message of the outbound and inboundmessages transmitted elsewhere on the two-way automatic communicationssystem, comprising: detecting a plurality of bits forming a preamble ofthe outbound message and extracting the plurality of bits, saiddetecting including detecting intermediate values included in a sequenceof the plurality of bits in the preamble; computing a ratio of signalstrength for bit locations in the preamble where one bit value isexpected to the signal strength for bit locations in the preamble whereanother bit value is expected; and, accepting the plurality of bits as avalid preamble for the outbound message if the computed ratio exceeds apredetermined value.
 2. The method of claim 1 in which the outboundmessage is imposed on an AC waveform and characteristics of the ACwaveform are removed from the plurality of bits by subtracting datacollected in one processing cycle from that collected in an earliercycle.
 3. The method of claim 2 further including checking the pluralityof bits extracted from the outbound message to determine if apredetermined threshold of signal strength is exceeded.
 4. The method ofclaim 3 further in which only a predetermined set of the plurality ofbits are required to exceed the predetermined threshold of signalstrength.
 5. The method of claim 1 in which the ratio R of the signalstrength of the bits with one value compared to the signal strength ofthe bits with the other value is defined as:$R = \frac{\sum\limits_{i \in Z}\; d_{i}}{\sum\limits_{i \in N}\; d_{i}}$where Z comprises a set of bit locations in the preamble where the bitsof the one value occur, N comprises a set of bit locations in thepreamble where the bits of the other value occur, and di is a receivedsignal strength at an index i; if the ratio does not exceed apredetermined threshold value, the plurality of bits are rejected as nota valid outbound message preamble.
 6. The method of claim 5 in which theplurality of bits have a value of 0 or 1 and the ratio R is the ratio ofzero bits to non-zero bit for the plurality of bits in the preamble. 7.The method of claim 5 in which a range of values for the ratio is 0.6 to1.0.
 8. The method of claim 2 in which an average signal amplitude forthe plurality of bits is computed for a plurality of possible points ofsynchronization and the ratio is determined using the average signalamplitude.
 9. The method of claim 8 in which the average signalamplitude is computed using a recursive filter.
 10. The method of claim8 in which the average signal amplitude is computed for each of fourpossible synchronization points and the ratio R of the signal strengthof the bits is defined as $R = \frac{y_{n,j}}{y_{n,k} + y_{n,l}}$ wherey_(n,j), y_(n,k), and y_(n,l) are the average signal amplitudes forvarious synchronization points of the plurality of possible points ofsynchronization on the waveform, where i,n,k,l are integers.
 11. Themethod of claim 10 in which the averages signal amplitudes of the fourpossible synchronization points are unweighted averages, created usingfinite impulse response (FIR) filters.
 12. The method of claim 10 inwhich the averages signal amplitudes of the four possiblesynchronization points are recursive averages, created using infiniteimpulse response (IIR) filters.
 13. The method of claim 9 in which anaveraging function for computing the averages signal amplitudes of thefour possible synchronization points is defined as:y _(n) =x _(n) +αy _(n−1) where α is an averaging constant x_(n) is aninput of the recursive filter y_(n−1) as a previous average signalamplitude and y_(n) is a current average signal amplitude.
 14. In atwo-way automatic communications system used by an electrical utility inwhich outbound messages are sent from the utility to a consumer andinbound reply messages are sent from the consumer back to the utility,the respective outbound and inbound messages being sent and receivedover the utility's power distribution system, a method for enabling atransponder to distinguish an outbound message from an inbound messageof the outbound and inbound messages transmitted elsewhere on thetwo-way automatic communications system, comprising: detecting aplurality of bits forming a preamble of the outbound message andextracting the plurality of bits, said detecting including detectingintermediate values included in a sequence of plurality of bits in thepreamble; computing a ratio of signal strength for bit locations in thepreamble where a “0” bit is expected to the signal strength for bitlocations in the preamble where a “1” bit is expected; and, acceptingthe sequence of plurality of bits as the preamble for the outboundmessage if: a) all of the “1” bits in the preamble are of a correctsign; b) the value of all the sequence of bits fall into a correct rangeof values for bit strength; and, c) a ratio${R = {\sum\limits_{i \in Z}\;{d_{i}/{\sum\limits_{i \in N}\; d_{i}}}}}\;$ exceeds a predetermined threshold value wherein Z comprises a set ofbit locations in the preamble where the signal amplitude is expected tobe near zero, N comprises a set of bit locations in the preamble wherethe signal amplitude is expected to be non-zero, and d_(i) an absolutevalue of the received signal at an index i, wherein i is an integer.